Saliva samples were taken and tested for cortisol and they were asked to play a computer game, which unknown to them, had been rigged.

As they played, critical messages from an unseen opponent-were flashed up on the screen. Researcher Dr Graeme Fairchild said: 'The game was rigged to be impossible. The whole point is to make them feel angry and annoyed, and as if they were being socially evaluated.'

Saliva samples were taken again afterwards and amounts of cortisol compared. As expected, levels of the hormone - which 'glues' memories into the brain - rose under stress in the boys without behavioural problems.

But they tended to fall in the unruly youngsters, the journal Biological Psychiatry reports.

The study, funded by the Wellcome Trust, suggests anti-social behaviour may be more biologically based than previously thought and that drugs could be developed to tackle it.